The present invention relates to a pressure gauge, and more particularly to a guage for measuring air pressure such as in an automobile pneumatic tire.
The piston-type pressure gauge is well known in the art, being a so-called dead weight apparatus in which the pressure to be measured is balanced by adjustment of a weight placed on the piston. Usually, the pressure readings are marked on the piston, and are read as the piston is moved out of its housing when the pressure is applied thereto. This is a very accurate type of gauge, employed for the calibration and testing of other gauges. However, the piston after non-use tends to stick within its housing, where care must be taken not to damage the piston which is usually always exposed.
Another important and widely used instrument is the Bourdon-tube pressure gauge, or spring-tube pressure gauge, in which pressure measurement is based on the deformation of an elastic measure element, such as a curved tube, by the pressure to be measured. The deformation is indicated by a pointer on a dial calibrated to give pressure readings. The tube which is of circular or oval cross-sectional shape, is closed at one end, and the pressure to be measured is applied to the other end, causing the radius or curvature of the tube to increase, i.e., the tube tends to straighten itself out.
In a diaphragm-pressure gauge, an elastic element is a stiff metallic diaphragm held between two flanges, the pressure is equal to the underside of the diaphragm, and movement of the latter is transmitted to a pointer. However, in both the Bourdon-tube and the diaphragm pressure gauge, a linkage must be used to connect the elastic element to the pointer. This linkage usually consists of a rack and pinion mechanism which is expensive to manufacture, and tends to be easily damaged if improperly handled. Additionally, the size of the pressure gauge must be increased to accommodate such a rack and pinion mechanism.